The biggest river by discharge volume is the Amazon at an impressive 209,000 cubic meters (7,380,765 cubic feet) per second. The Amazon drains a rainforest while the Mississippi drains much of the area between the Appalachian and Rocky Mountains, much of which is fairly dry.

The Amazon River in South America holds the undisputed record for the highest flow rate of any river on Earth, discharging an average of approximately 209,000 cubic meters per second (7,381,000 cubic feet per second) into the Atlantic Ocean. To put this into perspective, the Amazon's discharge is greater than the next seven largest global rivers combined and accounts for roughly 20% of all fresh water entering the world's oceans. The sheer volume of water is so immense that it creates a "freshwater plume" in the Atlantic that can be detected hundreds of miles from the river's mouth. This massive flow is driven by the vast Amazon Basin, which covers nearly 40% of the South American continent and receives staggering amounts of tropical rainfall throughout the year. During the peak rainy season, the flow rate can surge even higher, causing the river level to rise by over 30 feet and flooding vast areas of the surrounding rainforest. In 2026, scientists closely monitor this flow as a key indicator of the health of the global water cycle and the ongoing impacts of climate change on tropical ecosystems.